Good morning Simon,
I don't think that what you try to achieve can be achieved with the standard functionalities inside D365FO.
Why?
Because the signing limit policies cannot differentiate between OPEX and CAPEX.
You can bind the signing limits to document types, such as purchase order, invoice, etc. but you cannot further differentiate - for example - between a signing limit that applies for Capex invoices and one that applies for other Opex invoices.
You can of course try using workarounds such as the one suggested by Mattguo (two positions).
Rather than doing that, I would personally consider routing those approvals through MS PowerAutomate/Flow, which gives you a larger flexibility.
The setup is of course more complex as you would need to configure business events and the 'Flow' in addition but to me it looks like a more viable approach.
Alternatively, you can discuss a customization with a developer that would allow you differentiating between Capex and Opex in the signing limit policy form.
Best regards,
Ludwig